It is known that an access and/or aggregation network is a telecommunication network allowing to provide a number of users with telephone/data services. Typically, an access and/or aggregation network has a tree-like structure, so that the number of users is connected to a node (for instance, a router) of packet-switched core network. Traffic may be transmitted either in downstream direction, i.e. from the core network node to users, or in upstream direction, i.e. from a user to the core network node. Generally speaking, an access and/or aggregation network provides user segregation, i.e. users can not communicate one with the other, except in some particular cases, as it will be shown in further detail herein after.
Different technologies are known for implementing an access and/or aggregation network. Nowadays, a more and more frequent technology for implementing an access and/or aggregation network is the Ethernet technology. In particular, Ethernet technology provides implementation of a so-called “Virtual Local Area Network”, or briefly VLAN. A VLAN comprises a virtual set of nodes which are not necessarily part of a same network segment, but which are able to communicate as if they were part of a same network segment. In a VLAN, broadcast traffic generated by a node of a VLAN is broadcasted to all the other nodes belonging to the same VLAN. Further, as connections between nodes of a VLAN are Ethernet-based, they are bi-directional, i.e. they are able to support traffic in either direction.
An advantageous implementation of an Ethernet-based access and/or aggregation network is a so-called private VLAN. A private VLAN typically comprises the superimposition of a first VLAN, which is named primary VLAN, and at least a second VLAN, which is named secondary VLAN. The primary VLAN is adapted to transport downstream traffic. The secondary VLAN may be adapted to transport only upstream traffic; in this case, the secondary VLAN is an “isolated” secondary VLAN. Besides, as already mentioned, some particular services may require bi-directional communication between users. In this case, a secondary VLAN may be adapted to transport also bi-directional traffic between users belonging to a same community enjoying such services; is this case, the secondary VLAN is a “community” secondary VLAN. A private VLAN may comprise any number of secondary VLANs, with at most one secondary VLAN being an isolated VLAN.
Therefore, logically adjacent nodes of a same private VLAN are logically connected by a number of parallel bi-directional connections which is equal to the overall number of VLANs (both primary and secondary) comprised in the private VLAN.
Traffic direction of each VLAN and user segregation are implemented by suitably configuring interfaces between private VLAN and core network and interfaces between private VLAN and users. More particularly, three types of interfaces are provided, according to their configuration:                promiscuous interface: it is adapted to transmit traffic along the primary VLAN. Further, it is adapted to receive traffic from any other interface of the same private VLAN;        isolated interface: it is adapted to transmit traffic along the isolated VLAN. Further, it is adapted to receive traffic from promiscuous interfaces of the same private VLAN; and        community interface: it is adapted to transmit traffic along a community VLAN. Further, it is adapted to receive traffic both from promiscuous interface of the same private VLAN and from community interfaces of the same community VLAN.        
Typically, a private VLAN comprises a single promiscuous interface and a plurality of isolated or community interfaces. The promiscuous interface is adapted to interface the private VLAN with the core network node. Each isolated interface is adapted to interface the private VLAN with a user which is not subscribing any service requiring bi-directional communication between users. Each community interface is adapted to interface the private VLAN with a user which is subscribing a service requiring bi-directional communication between users.
When a service is provided by means of a private VLAN network, the service provider may be interested in monitoring traffic exchanged either between a user and the core network, or between two users.
The Ethernet technology provides a set of functions which is called OAM (“Operation, Administration and Management”) allowing to monitor a bi-directional connection. Implementation of such OAM functions is performed by transmitting particular messages, which are called OAM messages, along the connection to be monitored. Different types of OAM messages are provided, each type performing a different OAM function. OAM messages are typically formatted as Ethernet packets.
For instance, it is assumed to perform OAM monitoring of a point-to-point Ethernet connection comprising two end nodes and at least an intermediate node. For starting OAM monitoring, a first end node of the connection periodically transmits an OAM message along the connection. The OAM message passes through the intermediate node(s) and it is received by the second end node. In case the OAM message requires an OAM reply, the second end node, upon reception of the OAM message, transmits an OAM reply to the first node along the same connection. Then, the OAM reply is received by the first end node. Therefore, by processing the OAM message and the OAM reply, the first end node is capable of monitoring the connection in both traffic directions.